1. Field of the Invention
The present invention relates to an internal combustion engine with a cylinder having a pair of intake ports connected respectively through a pair of manifold passages to a duplex carburetor.
2. Prior Art
Devices for lean air-fuel mixture combustion and exhaust gas recirculation (EGR) have been employed for purifying exhaust gases or increasing mileage with a view to saving energy. Air-fuel mixtures to be burnt with such devices, however, are low in combustibility and unstable. To improve their combustibility, there have been proposed a pair of primary and secondary intake ports opening to an engine cylinder so as to supply air-fuel mixture only through the primary intake port when the engine rotates at low speed under light load. The mixture thus enters through a relatively restricted area across one of the ports, causing swirls in the cylinder that help increase the rate of combustion.
The air-fuel mixture that flows through the secondary intake port is controlled by a valve disposed in a secondary intake passage leading to the secondary intake port, the valve being opened and closed by a mechanism that is energizable by a vacuum signal available at the intake passage. Such conventional mechanisms have the disadvantage that when the engine rotates at low speed under heavy load, or in the low intake flow zone, the throttle valve is opened, making the pressure inside the intake passage substantially atmospheric. Then, the control valve in the secondary intake passage is forced to open despite the low speed of rotation of the engine. This results in an increased intake area and a reduced velocity of flow of the mixture into the combustion chamber. Accordingly, no swirls are created within the cylinder, with the results that the mixture combustibility is impaired, purification of exhaust gases fails, and reduced fuel consumption is not obtained.